Machines for packaging pourable food products, such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc., are known, in which the packages are formed from a continuous tube of packaging material defined by a longitudinally sealed web.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material comprises a layer of barrier material, defined for example by aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material defining the inner face of the package eventually contacting the food product.
To produce aseptic packages, the web of packaging material is unwound off a reel and fed through an aseptic chamber, in which it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide which is later evaporated by heating, and/or by subjecting the packaging material to radiation of appropriate wave length and intensity. The sterilized web is then folded into a cylinder and sealed longitudinally to form, in known manner, a continuous, longitudinally sealed, vertical tube. In other words, the tube of packaging material forms an extension of the aseptic chamber, and is filled continuously with the pourable food product and then fed to a forming and (transverse) sealing unit for producing the individual packages, and in which it is gripped between pairs of jaws, which seal the tube transversely to form pillow packs which are then separated by cutting the seal between the packs.
More specifically, the tube portion compressed between the jaws is simultaneously sealed crosswise by heating, e.g. induction or ultrasonic heating, means carried by the jaws themselves. Once sealing is completed, a knife is activated to cut the tube of packaging material along the centre of the sealed portion and detach a pillow pack from the bottom end of the tube. The bottom end is therefore sealed crosswise, and the jaws open to avoid interfering with the tube and the other pair of jaws. At the same time, the other pair of jaws, activated in the same way, moves down from a top dead-centre position, and repeats the above gripping/forming, sealing and cutting operations.
The pillow packs are then conveyed to a finish folding station, where they are folded mechanically to form the finished packages.
Known units also comprise, for each pair of jaws, two facing forming flaps hinged to the jaws and movable between a withdrawn or open position, and a forward or closed position in which they mate, when the jaws are closed, to define a cavity defining the shape and volume of the package to be formed between them.
In one known solution, the closing movement of the forming flaps is controlled by cams fixed to the machine structure, and which are specifically sized and located to produce a given type of package, and interact with respective rollers carried by the tabs.
Machines of the above type have been extremely successful commercially, and have proved extremely reliable, to the extent of requiring very little maintenance, even after many years' service.
On the other hand, they have several drawbacks caused, in particular, by being fairly rigid production-wise.
That is, machines of the above type can be adapted to produce packages of different volumes, but only at the expense of major alterations to the machine, which include replacing the forming flaps on the jaws, replacing all the parts, even static (such as the cams), controlling the closing movement of the tabs, and subsequently adjusting the new system. In addition to the manufacturing cost of the new component parts, such alterations therefore also involve a good deal of downtime.
One solution to the above problem is described in EP-A-1 101 700, which describes a form-and-seal unit in which the closing movement of the forming flaps onto the tube of packaging material is controlled by cams carried by the forming flaps themselves and interacting with rollers fixed to the unit structure, so that the volume of the packages produced can be modified by simply changing the forming flaps (which, being designed for a specific type of package, must be changed anyway at each production change) with no work needed on the static parts of the machine.
Even the above solution, however, is not without drawbacks. That is, the fact that the cams controlling the closing movement of the forming flaps are fitted to the tabs themselves increases the weight of the moving component parts, and so creates dynamic problems and poses limits to the output rate. Moreover, the geometry of the system poses serious restrictions on the movement to prevent interference by the cams.